Seedlings for transplanting purposes have long been grown in various types of containers. Depending on the species and its intended use, the method provides various advantages. The containers, or cells, can be combined to form trays of sizes appropriate for each individual species to be processed. A good cultivation tray, particularly one that is intended for forestry purposes, should possess good handling properties in different processing stages, good growing properties like correct size of lump, good oxygen supply for the roots, good lump release properties, and should provide for draining of excess water.
Particularly in the cultivation of woody species, problems may occur by cell cultivation. A root emerging from a growing seedling or cutting tends to grow radially outwards toward the cell wall. As the root reaches the curved cell wall, it will follow the wall continuing its linear growth, growing in a descending spiral until the cell bottom is reached, whereby the root may further continue its circular growth. This results in a root system which may be harmful to the plant after transplanting. The roots do not provide good structural support; the spiralling roots tend to grow downwards to set at a depth where oxygen supply is insufficient. The intertwined roots also impair the flow of nutrients, as they increase in thickness.
Numerous methods have been devised to restrain root spiralling. Cell walls may be impregnated with a chemical compound that stops root growth as the root tip reaches the cell wall, as disclosed in, e.g. Finnish Patent 71650. This method is feasible, e.g. in single use flexible cell systems made of paper or the like. When rigid materials, e.g. plastics are used, one possibility is to design undulating cell walls, providing pockets to intercept the root tips, whereby these stop growing. Such solutions are disclosed in, e.g. U.S. Pat. Nos. 4,716,680 and 4,442,628.
Root growth is halted also when the root tip meets free air space, which phenomenon is known as air pruning. Thus, cells having side wall apertures of various shapes and sizes have been developed. For example, in Swedish Patent 8001210-7 a cell tray is disclosed where the cells comprise an upper framework and ribs protruding downwards therefrom, the ribs being sufficiently rigid to retain the lump of growth medium. Openings or slits can be combined with other structural features of the cell wall to direct roots toward the openings, as disclosed in e.g. U.S. Pat. Nos. 4,497,132, 4,510,712 and 5,241,784.
When the linear growth of a root is halted, branching of the root commences. It has been observed that following transplanting, development mainly takes place in the roots that have already been formed in the cell cultivation stage. Thus, the number and distribution of root tips are crucial for the success of a transplanted seedling.
For the development of several plants, in particular northern coniferous wood species, it is essential that roots growing close to the ground surface develop well. Near the surface supply of oxygen and nutrients is abundant, and for small seedlings it is deleterious that the root systems develops at too deep a level, where the soil is cold.
It is a common feature of all prior art nursery cells that the upper edge of the cell is unbroken, which is an obvious solution from a structural point of view. However, it has been observed that root spiralling can occur also very close to the lump surface. As water is supplied from above, surface roots develop. Even a narrow rim section of a cell, being 5-10 mm high, may induce spiralling of surface roots. A surface root growing into an obstacle may turn to grow vertically into a so-called "knee root", whose tip sets too deep for being useful for the plant; or the root may grow over the cell edge into an adjacent cell, which makes subsequent handling of the seedling more difficult, as well as impairs its later development.
U.S. Pat. No. 4,510,712 discloses a cell tray having slits reaching close to the upper frame-work. However, said upper framework forms an unbroken top surface on the tray. A prerequisite for efficient air pruning is sufficient venting between cells in a tray. Therefore, air pruning trays are provided with vertical air channels at least in the junctions between cells, as disclosed in e.g. U.S. Design Nos. 322,048, 325,714 and 401,530.